runtime: add gc layout info for some basic types

src/reflect/type.go

@@ -64,6 +64,7 @@
 package reflect
 
 import (
+	"internal/gclayout"
 	"internal/itoa"
 	"unsafe"
 )
@@ -961,6 +962,26 @@ func (t *rawType) Align() int {
 	}
 }
 
+func (r *rawType) gcLayout() unsafe.Pointer {
+	kind := r.Kind()
+
+	if kind < String {
+		return gclayout.NoPtrs
+	}
+
+	switch kind {
+	case Pointer, UnsafePointer, Chan, Map:
+		return gclayout.Pointer
+	case String:
+		return gclayout.String
+	case Slice:
+		return gclayout.Slice
+	}
+
+	// Unknown (for now); let the conservative pointer scanning handle it
+	return nil
+}
+
 // FieldAlign returns the alignment if this type is used in a struct field. It
 // is currently an alias for Align() but this might change in the future.
 func (t *rawType) FieldAlign() int {

src/reflect/value.go

@@ -1479,7 +1479,8 @@ func MakeSlice(typ Type, len, cap int) Value {
 	ulen := uint(len)
 	ucap := uint(cap)
 	maxSize := (^uintptr(0)) / 2
-	elementSize := rtype.elem().Size()
+	elem := rtype.elem()
+	elementSize := elem.Size()
 	if elementSize > 1 {
 		maxSize /= uintptr(elementSize)
 	}
@@ -1493,7 +1494,9 @@ func MakeSlice(typ Type, len, cap int) Value {
 	var slice sliceHeader
 	slice.cap = uintptr(ucap)
 	slice.len = uintptr(ulen)
-	slice.data = alloc(size, nil)
+	layout := elem.gcLayout()
+
+	slice.data = alloc(size, layout)
 
 	return Value{
 		typecode: rtype,